Measuring diameters of objects in the solar system is difficult due to their vast distances and huge sizes. If the distance to an object is known, we can use a method of indirect measurement in which we measure the angular diameter, and then apply a geometric principle called similar triangles.

This Reading provides background information on the underlying concepts in the Activities included in the book, Project Earth Science: Astronomy, Revised 2nd Edition. The Readings go beyond what is found in the background information in the Activities. They can be used as supplementary information for the teacher, or to enhance classroom discussions.